This invention relates generally to separating wafers (or similar structures) from a stack of wafers. More particularly, this invention relates to separating wafers without damaging contact with their edges, thereby eliminating breakage otherwise caused by such contact.
The manufacture of semiconductor parts, or xe2x80x9cchips,xe2x80x9d typically begins with the processing of wafers made from silicon, gallium arsenide, or other semiconductor compounds. Initially, a crystalline (or polycrystalline) boole is sawn simultaneously into several hundred thin wafers using means such as a wire frame saw. The wafers are typically quite thin, of the order of 200 microns thick, and are most often of circular cross-section. This sawing produces a stack of wafers contaminated with sawdust and sawing fluid. The wafers must then be removed from the stack, cleaned, and placed into a product carrier such as a cassette or tray before further processing.
Removing the wafers from the stack is typically done manually because the wafer material is brittle. Rough handling can easily result in breakage, or chipping of the edges, which would render the wafer useless for further manufacture. To prevent such damage, vacuum xe2x80x9cwandsxe2x80x9d are commonly employed as a means for manually lifting individual wafers from the stack. A wand typically consists of a stem with an internal channel for vacuum, a broad tip, and a vacuum actuator switch for connecting the stem to a vacuum source. An operator picks up a wafer by placing the broad tip of the wand in the center of the planar surface of the wafer, thereby allowing the vacuum to cause the wafer to adhere to the wand. The operator can then transport the wafer from one location to another, and can then release the wafer by shutting off the vacuum to the wand.
Although adequate, manually separating wafers has a number of drawbacks. The wafers in a stack have a tendency to stick to one another, primarily due to surface tension effects. A wand alone cannot overcome these effects, requiring the operator to slide the wafers apart by pushing against their edges. This handling can damage the wafers. Furthermore, the labor required is a significant processing cost and takes considerable time.
Attempts to automate the separating process have been tried but not widely adopted. U.S. Pat. No. 5,213,451 to Frank et al. describes the use of a dam and jets of fluid, such as water or oil, to separate wafers in a stack. The jets push the outermost wafer up and over the dam while the wafers beneath are restrained in the stack by the dam. A feed unit gradually lifts the wafer stack, causing each wafer to eventually be pushed over the dam by the fluid jets. While this method is faster than manually separating the wafers, it still exposes them to potential damage. The fluid jets cause the outermost wafer to slide against the adjacent wafer and drive the thin edges of the other wafers against the dam, either action of which can cause wafer breakage.
An objective of the invention, therefore, is to separate the wafers in a stack without causing sliding contact between the wafers. Another objective is to separate the wafers without striking the wafer edges with a force sufficient to damage them.
In accordance with the invention, a novel method for individually separating wafers from a stack of wafers is disclosed. Briefly, the method comprises directing multiple jets of fluid between an outermost wafer in the stack and an adjacent wafer. The jets are at sufficient pressure and at sufficiently spaced-apart locations around the wafer stack to cause the outermost wafer to separate longitudinally from the adjacent wafer without lateral movement there between. The method further includes attaching to a planar surface of the outermost wafer and moving the attached outermost wafer and wafer stack relatively apart, thereby separating the wafers in the stack without contact between a solid object and a wafer edge.
An apparatus in accordance with the invention is also disclosed. The apparatus includes a base and a chuck attached to the base. The chuck is adapted to attach to a planar surface of an outermost wafer in a stack. A plurality of nozzles is adapted to direct multiple jets of fluid between the outermost wafer and an adjacent wafer. The jets are at sufficient pressure and at sufficiently spaced-apart locations around the wafer stack to cause the outermost wafer to separate longitudinally from the adjacent wafer without lateral movement there between. The nozzles can be attached to the base or be separated therefrom, such as on a container that surrounds the wafer stack.
These and other aspects of the invention are described below with respect to several embodiments thereof.